1. Field of the Invention
The present invention relates to an antenna unit for high-frequency use, and more particularly, it relates to an antenna unit whose resonance frequency is switchable so that the same can be employed in a plurality of frequency bands.
2. Description of the Background Art
A smaller antenna unit is required for a mobile communicator. An inverted-F antenna unit is known as a type of miniature antenna unit which can be applied to such use.
An exemplary inverted-F antenna unit is described in "Small Antennas" by K. Fujimoro, A. Henderson, K. Hirasawa and J. R. James, Research Studies Press Ltd., England. An example of such an inverted-F antenna unit is now described with reference to FIG. 1. Referring to FIG. 1, an inverted-F antenna unit 1 has a rectangular metal plate 2 which serves as a radiating part. One side edge of the metal plate 2 is bent to be perpendicular to the metal plate 2, thereby forming a ground terminal 3. Another side edge of the metal plate 2 is also partially bent to form a feed terminal 4.
Due to the aforementioned structure, it is possible to mount the inverted-F antenna unit 1 on a printed circuit board by inserting the ground terminal 3 and the feed terminal 4 in through holes which are provided in the printed circuit board.
In conventional miniature antennas including the aforementioned inverted-F antenna unit, however, the bandwidth is so insufficient that the antenna can cover only a transmission or receiving side frequency band in application to a mobile communicator. But as shown in FIG. 2, when frequency bands Tx and Rx of transmission and receiving sides are separated from each other by a frequency A in a portable mobile communicator, a single antenna unit must have a bandwidth B, to enable both transmission and receiving. However, the conventional miniature antenna unit cannot satisfy such a bandwidth B.
In a system provided with transmission and receiving sides having the same frequency bandwidth such as the PHP (personal handy phone) system, it is possible to cover both the transmission and the receiving frequencies with the conventional miniature antenna unit for a mobile communicator. However, there has been no miniature antenna unit which can cover both the transmission and the receiving frequency bandwidths in a system provided with different transmission and receiving frequencies.
Thus, development of a miniature antenna unit whose resonance frequency is switchable has been awaited.